Integrate tests from MarkDown Test Suite

[Cerdic]

A MarkDown TestSuite has been initiated by Karl Dubost
https://github.com/karlcow/markdown-testsuite

It would be interesting to integrate it and be sure ParseDown is compliant to this test suite.

[JCMais]

Looks like we only need a cli interface to make it work. I will take a look into that.

[aidantwoods]

As mentioned in #99

We now test against (but do not currently pass all of) the CommonMark examples. CommonMark is a bit more specific than John Gruber's original Markdown spec, so I think that testing against John Gruber's suite is probably no longer needed :)

Looks like the test suite linked ceased development in favour of CommonMark :)